Most folks see a wood stork in this position and are startled to see that their knees bend backwards. They don't. What is mistaken for their knees as actually their ankles, the knee is up close to their body so it's an easy mistake to make. The way I suggest you look at wood storks is that they walk on their toes. Once you visualize them walking on their toes and that what's mistaken for the knee is actually their ankle it becomes easier to understand.
Seeing a wood stork in this position confuses most folks.